1. Field of the Invention
The present invention relates to high-frequency couplers which are interposed between a power amplifier and an antenna in a radiofrequency transmission chain. Such couplers are used to extract a portion of the signal transmitted to the antenna to, for example, measure its amplitude and accordingly adapt the amplifier gain.
2. Discussion of the Related Art
FIG. 1 partially and schematically shows a conventional example of a radiofrequency transmission circuit comprising a coupler 1 of the type to which the present invention applies.
The transmit circuit is generally contained in a circuit or radiofrequency front end transmit-receive circuit comprising a receive chain and a transmit chain from a same antenna 2. In FIG. 1, only the transmit circuit is considered, a separator (not shown) being generally provided at the input of antenna 2 to make out the transmitted signals from the received signals.
A radiofrequency signal (RF) carried by a frequency in a predefined band is applied to the input of a power amplifier 3 (PA), the output of which is connected to antenna 2. Coupler 1 is interposed between amplifier 3 and antenna 2 to sample a signal proportional to the transmitted signal. The signal extracted by coupler 1 is, for example, provided at the input of a detector 4 (DET), generally a peak value or mean value detector, in charge of measuring the signal intensity. An output of detector 4 is combined (comparator 5) with a required power threshold LV to provide a level control signal DV to amplifier 3. Coupler 1, detector 4 and comparator 5 form a loop for controlling the power of the transmit amplifier by reference LV.
Reference LV may, depending on the application, be predetermined or be provided by circuits external to the transmit-receive system. For example, in an application to mobile telephony, the required power level is generally provided upon completion of the communication, then periodically, by a beacon to which the unit is connected. In fact, upon completion of a communication, the transmit circuit transmits at full power. This power is measured by the beacon receiving the signals, which in turns transmits, for the phone, a power reference value for the rest of the communication.
FIG. 2 schematically shows a conventional structure of a coupler 1 used in a transmit chain such as illustrated in FIG. 1. Said coupler is an electromagnetic coupler using two tracks 6 and 7 generally patterned on a printed or integrated circuit and coupled to each other. One end of a first track 6 is connected to the output of amplifier 3 while another end 8 is intended to be connected to antenna 2 (generally via the band separator). A second track 7 has a first end connected to the input of detector 4 and a second end connected to ground, generally by a resistor R. The shape given to track 6 within coupler 1 is, in plane view, that of an arch or arches inside of which is housed, in parallel sections, second track 7. The running of a signal in first track 6 generates, by induction, a proportional signal in second track 7. This signal can then be measured by detector 4.
A disadvantage of an inductive coupler such as illustrated in FIG. 2 is that it is frequency-selective. Indeed, the coupler is sized according to the frequencies that it must be able to extract. In practice, in an application to phone, this results in having to provide a coupler for the so-called DCS or PCS band around 1800 MHz and a coupler for the so-called GSM band around 900 MHz. The use of two couplers adversely affects the desired miniaturization of radiofrequency transmit systems. In practice, given the used frequencies, the tracks of an inductive coupler have several centimeters (for example, on the order of 3 cm) of expanded length. Further, if the surface area taken up by the coupler is desired to be reduced, a problem of thickness of the coupler tracks arises.